It is well known that the signals transmitted by the Global Positioning System (GPS) satellites can be used by a GPS receiver for navigation. In order to effectively operate a navigation system using the GPS satellite signals, we need to have at least four satellites in view at all times. This is almost never a problem in the open view. However, since the GPS satellite signals are weak microwave signals (1575.42 MHZ and 1227.6 MHZ), they cannot penetrate through obstacles. Thus, if the GPS receiver antenna is surrounded by tall structures or inside a building, there is no direct line-of-sight reception of the satellite signals. In such a case, navigation using the GPS satellite signals is not possible.
There are future applications where there will be a need to navigate inside buildings. One example is the location tracking of a portable valuable object that may be inside or outside a building. Currently, location tracking of railroad cars and buses are being effected with GPS receivers attached to them. As the price and size of GPS receivers decrease, it will be feasible to attach a GPS receiver to any small movable object of high value. This will be a future method for keeping track of the location of valuable objects or to find an object of interest inside or outside a building. It will be possible to locate the object of interest by simply interrogating the attached GPS receiver and having it send its positional coordinates over a radio link. The GPS receiver can retain coordinates of its last position or compute coordinates of its new position when interrogated. This will be particularly useful for locating a missing object. Presently, this cannot be done if the GPS receiver is inside a building because it cannot track the weak GPS satellite signals indoors. It is desirable to use the same GPS receiver that tracks GPS satellite signals outdoors to also track similar signals indoors, since this would not require a hardware change of the existing GPS receiver.
The present invention proposes a system for navigating inside a building using an existing GPS receiver. Since this system is based on the same positioning method that underlies a standard GPS navigation system, an understanding of how the GPS satellite signals are used for navigation is needed. The following discussion will provide the background.
The Global Positioning System (GPS) is a satellite-based navigation system implemented and controlled by the U.S. Department of Defense. The GPS has three segments: the Space Segment consisting of the GPS satellites, the Control Segment consisting of a Master Control station located in Colorado and monitor stations located around the world, and the User Segment consisting of the GPS receivers. Currently, the GPS Space Segment consists of 24 satellites or space vehicles (SVs): 21 navigational SVs and 3 active spare SVs orbiting the earth in 12-hour orbits. The orbit altitude (about 20,200 km above the earth) is such that the SVs repeat the same track and configuration over any point approximately every 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about 55 degrees with respect to the equatorial plane. This constellation provides a user with five to eight SVs visible from any point on the earth.
The satellites continuously send signals down toward the earth in the form of radio waves. If a user is equipped with a GPS receiver and if enough GPS satellite signals are penetrating the local surroundings, the user is able to determine his location by extracting positional information from the GPS satellite signals. For military reasons, the Department of Defense purposely degrades the accuracy of the GPS positional information by making the signals from any GPS satellite vary with time, introducing errors of as much as 100 meters in a 30 second period. This degradation is called Selective Availability which has been adopted primarily to prevent terrorists from using GPS as a bombing aid.
GPS satellites transmit information on two carrier signals: L1 band centered at 1575.42 MHZ and L2 band centered at 1227.6 MHZ. Each satellite has four atomic clocks, one is for timing and synchronization while the other three are for redundancy in case of failure. The timing clock ticks at the basic frequency of 10.23 MHZ. This basic frequency is multiplied by 154 and 120 to generate the frequencies of L1 and L2 carrier signals respectively. The L1 carrier signal is then modulated by three binary signals: a Coarse Acquisition (C/A) code, a Precision (P) code, and a Navigation Message signal. The C/A code and the P-code are pseudo-random noise (PRN) code sequences, i.e., noise-like signals. The P-code is a very long (about seven days) 10.23 MHZ PRN code sequence available for military use while the C/A code is a 1.023 MHZ PRN code sequence available for civil navigation. The Navigation Message is a 50 Hz signal which includes the ephemeris which provides orbital information of the satellite in question, clock information and other data related to the GPS satellite constellation.
If the clocks provided in the GPS satellites and the clock in a GPS receiver located at a survey point are synchronized with each other, the propagation delay time of the GPS radio waves transmitted by a GPS satellite is the difference between the time at which they are transmitted and the time at which they are received by the GPS receiver. By multiplying this propagation delay time by the speed of light, the distance between the survey point and the corresponding satellite can be calculated. With the knowledge of this distance and the three-dimensional coordinates of the satellite, derived from the ephemeris of its C/A code, the survey point can be pictured as a point located somewhere on the surface of an imaginary sphere having as center the position of the satellite and as radius the distance between the satellite and the survey point. With two satellites, there are two such imaginary spheres, and the survey point can be pictured as a point located somewhere on the circle where the two spheres intersect. With three satellites, there are three corresponding imaginary spheres which intersect at two locations. One of the locations is the correct answer for the position of the survey point, the other location would put the survey point at an impossible altitude or would indicate that the survey point is moving at an impossible velocity. Thus, theoretically we could disregard one of the two locations and proceed with only measurements from three satellites. However, due to inherent imperfect synchronization between the satellites' clocks and the GPS receiver's clock, measurements from four satellites are needed. With four satellites, there are four imaginary spheres which intersect each other at a single location, and the three-dimensional coordinates of the survey point can be calculated as the coordinates of that location.
The above positioning method provides position information with an accuracy of approximately 100 meters which includes the effect of Selective Availability. Differential GPS is a technique which uses differential corrections to obtain an accuracy of 2 to 5 meters. An accuracy of 1 meter can be achieved by using Differential GPS in conjunction with a state estimator such as a Kalman filter. Very high accuracy can be obtained with a more complex technique called Carrier Phase.
To implement the Differential GPS technique, there must be a fixed known location called the reference point or base. When the base computes its position from the GPS satellite signals, it compares the solution to the known reference values and transmits the error via some wireless communication link. A GPS user out on the road receives these differential corrections and adds them to its own computed solution to obtain more accurate positional coordinates. This Differential GPS technique works because the satellites are so far above the earth that positional errors measured by one GPS receiver are almost exactly the same for any other GPS receiver in a relatively small area.